Mooring lines and tension legs are generally made from steel link chain cables or polyester ropes having a cross sectional area of up to 750 cm2. In service they carry tensile loads for long periods while submerged in sea water. The weight of steel in sea water is 92 percent of its weight in air. Therefore, due to the weight of the steel chains, the buoyancy of the offshore platforms fixed to the sea floor by such chains must be larger than otherwise required so they can buoy the lines.
Transport and placement of steel mooring chains and tension legs is difficult due to their length and weight. Typically they are transported by ship or rail to a nearby port, and offloaded to very expensive heavy lift crane vessels or special anchor handling vessels for transportation and offshore installation. If their weight and bulk could be reduced substantially and their ability to be lengthened and shortened readily could be improved, then they could be assembled to a predetermined length and more, easily transported, handled, and more rapidly installed with less expensive and more readily available support vessels.
It has been proposed to use ropes of polyethylene fibers, such as the Dyneema® fiber of DSM. The offshore industry is already using polyester ropes for deepwater mooring applications. Such materials are approximately neutrally buoyant in sea water. The tensile strength of such materials is sufficient for long term mooring design. However, ropes have the drawback that they cannot be easily gripped, since their outer covering gets torn off, nor can they be held in place by chain stoppers. Ropes are also sensitive to the abrasive action of mud and sand particles which may penetrate and cause wear between the ropes fibers, thereby weakening of the rope. For these reasons it is often preferred to use metal chain links.
As opposed to fiber ropes, chain links can be held in place by chain stoppers. The chain stoppers can be used to secure the chain at a specific lengthy thereby adjusting the tension and optimizing the related station keeping performance. Typically, a chain stopper has two latches holding the chain in place, bearing upon the shoulders of a single link. A chain is pulled through the chain stopper until the desired position, chain angle and chain tension is obtained. An example of a chain stopper is for instance disclosed in U.S. Pat. No. 7,240,633.
Under axial load, the individual chain links are subjected to all forms of primary stresses, i.e. bearing, bending, shear and tensile stresses. Near the contact points between links, the bearing load due to axial tension is transformed into complex stress patterns that result in the highest stress in the bar at symmetric locations roughly +/−45 degrees on either side of the crown. Otherwise, for a normal steel chain link, much of the steel structure is highly underutilized. This is because the existing manufacturing processes and machinery using forged round bar stock are well embedded into the traditional chain making industry, resulting in very little advancement in the chain geometry or utilization of hybrid solutions. This is particularly the case when a link is held in a chain stopper. Due to cyclic loads, the chains are also susceptible to fatigue failure. In addition, during transport or installation of the chain the individual links may be subjected to high impact loads.
The complicated stress pattern within the individual chain links when the chain is under tensile load hinders a straightforward use of fibers or fiber reinforced material. In fibers, the greatest strength results when the direction of the fibers is in the direction of the load. Unidirectional composite materials have relatively low shear strength parallel to the fiber direction. Link-to-link attachments cause large stresses in the composite matrix in directions having inherently low strength.
U.S. Pat. No. 5,269,129 discloses a chain formed of links made of fiber-reinforced resin composite material. Each link has a terminal loop located at each axial end of a long strap. Loops, located at adjacent ends of successive links, are joined by relatively short connecting links that overlap bushings located within each of the loops. The bushings and connecting links are held in position at each lateral side of the links by pins and washers. A ring surrounds each link where the strap flares to form each terminal loop. The loops may be unitary or spaced laterally to receive within the space a unitary loop of an adjoining link aligned axial Iy with the other loop. A pin located within the loops supports washers at each lateral side of the links to maintain the position of the links and to transfer load between the links. Such chains have the drawback that they have only moderate impact resistance. The links comprise a number of washers, pins and other separate parts resulting in an elaborate to assembling of the chain. Moreover, the strength of the chain is determined by the strength of the pins linking the chain links. The chain links are shaped rather differently from the traditional interlocking toroid steel chain links, so their use requires modification of existing equipment and facilities, such as chain stoppers.